Internal antennas have been used for some time in portable radio communication devices. There are a number of advantages connected with using internal antennas, of which can be mentioned that they are small and light, making them suitable for applications wherein size and weight are of importance, such as in mobile phones.
However, the application of internal antennas in a mobile phone puts some constraints on the configuration of the antenna element, such as the dimensions of the element, the exact location of feeding and grounding portions etc. These constraints may make it difficult to find the correct tuning and matching of the antenna. This is especially true for so-called multi-band antennas, such as double-band antennas, wherein the antenna is adapted to operate in two or more spaced apart frequency bands. In a typical dual band phone, the lower frequency band is centered on 900 MHz, the so-called GSM 900 band, whereas the upper frequency band is centered around 1800 or 1900 MHz, the DCS and PCS band, respectively. If the upper frequency band of the antenna device is made wide enough, covering both the 1800 and 1900 MHz bands, a phone operating in three different standard bands is obtained.
The European patent publication EP 1 003 240 A2 discloses a surface mount antenna comprising first and second radiation electrodes separated by a gap. Each electrode is connected to a grounded connection electrode, providing a double resonance with two pass bands. The two pass bands overlap slightly, effectively creating a single-band antenna with one wide pass band instead of a double-band antenna. No guidance of how to obtain desired antenna characteristics is given.
The European patent publication EP 1 067 627 A1 discloses a dual band radio apparatus comprising a first and a second antenna element, both connected to a ground plate. A capacitive coupling is provided between the two antenna elements.
IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, describes in an article by Liu Z D et al. “Dual-Frequency Planar Inverted-F Antenna”, pp 1451-1458 a dual-band antenna. An antenna with a single-input port is described on page 1457, where it is indicated that the dual-band antenna can also work with a single feed by electrically shorting the two radiating elements using common short pins.